The process of preparing optical or ophthalmic lenses begins usually with an unfinished or semi-finished glass or plastic lens blank. Typically a semi-finished lens blank has a finished polished front surface and an unfinished back surface. By grinding away material from the back surface of the lens blank the required corrective prescription is generated. Thereafter the surface having had the corrective prescription imparted thereto is polished and the peripheral edge of the thus processed lens blank is provided with a final desired contour thereby establishing a finished optical or ophthalmic lens. According to other processing methods, the finished ophthalmic lens can be directly processed from a lens blank using for example three directional machining. The lens blank can be either a plastic or a glass lens blank.
It is necessary during these various processing operations to securely maintain the lens blank in accurate alignment and in place on the holding unit. This procedure is often referred to as “lens blocking”.
Heretofore various materials were employed to secure the lens blank. These materials include glues, pitch and low temperature fusible metal alloys. The use of glues and pitch suffers the disadvantage of generally being non-reusable or non-reclaimable. The use of low temperature metal alloys caused significant environmental and health hazards especially since these alloys were most often fabricated from such metals as cadmium, tin, lead and bismuth.
To overcome these issues, organic low shrinkage materials have been developed to be used as lens blocking materials.
U.S. Pat. No. 6,036,313 in the name of 3M Innovative Properties Company discloses examples of compound families suitable for lens blocking with thermoplastic materials.
Preferred thermoplastic compositions are non-toxic, environmentally safe, preferably biodegradable and may be recycled. The thermoplastic material is solid at room temperature and has a low melting or softening point.
Blocking an optical lens on a holding unit thanks to a thermoplastic material layer may be obtained by providing a molten or softened thermoplastic material on an assembling surface of the holding unit and pressing the lens to be blocked.
Gaskets may be provided around the holding unit to limit the flow of the molten or softened thermoplastic material when pressing the lens.
U.S. Pat. No. 6,012,965 a method for blocking an ophthalmic lens blank to a support block.
Previous blocking methods are not fully satisfactory because this may lead to undesired positioning errors and/or insufficient blocking of the lens.
Accordingly, there remains a need for improving blocking a lens secured to a holding unit thanks to a thermoplastic material.